To determine the effectiveness of vigorous realimentation with dietary fat, 17 subjects aged 64.0 +/- 2.1 years (mean +/- 1 SEM) were pump-fed via a nasogastric tube for an average of 22 days. The diet was liquid and nutritionally complete, high in unsaturated fat (67% of energy) and in the caloric density (12.6 kJ/mL or 3 Kcal/mL) [corrected]. Advanced malnutrition was manifested by 74% of the ideal body weight, subnormal anthropometric measurements, and low serum protein levels. At an intake of 17,986 +/- 945 kJ (4068 +/- 225 Kcal [corrected]) and 344 +/- 18 g of fat per day, the rate of nutrient absorption was 93% for energy and fat and 88% for protein. An increase in the daily fecal fat to 23 +/- 6 g was not associated with diarrhea. While serum triglyceride levels remained unchanged, the total cholesterol level decreased, with a relative increase in the high-density lipoprotein level. Effective utilization of nutrients resulted in a positive nitrogen balance and increases in body weight, triceps skinfold, the midarm muscle circumference, total iron binding capacity, and serum urea nitrogen level.